Method for producing a ultra high strength coated or not coated steel sheet and obtained sheet

ABSTRACT

A method for producing a cold rolled steel sheet having a tensile strength ≥1470 MPa and a total elongation TE≥19%, the method comprising the steps of annealing at an annealing temperature AT≥Ac3 a non-treated steel sheet whose chemical composition contains in weight %: 0.34%≤C≤0.40%, 1.50%≤Mn≤2.30%, 1.50≤Si≤2.40%, 0&lt;Cr≤0.5%, 0&lt;Mo≤0.3%, 0.01%≤Al≤0.07%, the remainder being Fe and unavoidable impurities, quenching the annealed steel sheet by cooling it to a quenching temperature QT&lt;Ms transformation point and between 150° C. and 250° C., and making a partitioning treatment by reheating the quenched steel sheet to a partitioning temperature PT between 350° C. and 420° C. and maintaining the steel sheet at this temperature during a partitioning time Pt between 15 seconds and 120 seconds.

This is a Divisional of U.S. patent application Ser. No. 15/323,232, filed Dec. 30, 2016 which is a National Phase of International Patent Application PCT/IB2014/002379, filed Jul. 3, 2014, the entire disclosure of which are hereby incorporated by reference herein.

The present invention concerns the manufacture of coated or non-coated high strength steel sheet having improved tensile strength and improved total elongation and the sheets obtained by this method.

BACKGROUND

To manufacture various equipment such as parts of body structural members and body panels for automotive vehicles, it is now usual to use bare, electro-galvanized, galvanized or galvannealed sheets made of DP (dual phase) steels multi-phase, complex phase or martensitic steels.

For example, a high strength multi-phase may include a bainite-martensitic structure with/without some retained austenite and contains about 0.2% of C, about 2% of Mn, about 1.5% of Si which would result in yield strength of about 750 MPa, a tensile strength of about 980 MPa, a total elongation of about 10%. These sheets are produced on continuous annealing line by quenching from an annealing temperature higher than Ac3 transformation point, down to an overaging temperature above Ms Transformation point and maintaining the sheet at the temperature for a given time. Optionally, the sheet is galvanized or galvannealed.

To reduce the weight of the automotive parts in order to improve their fuel efficiency in view of the global environmental conservation it is desirable to have sheets having improved strength-ductility balance. But such sheets must also have a good formability.

SUMMARY OF THE INVENTION

An object of the present invention provides producing sheets made of steel using so called quenched and partitioned having improved mechanical properties and good formability. Coated or non-coated (bare) sheets having, a tensile strength TS of about 1470 MPa and a total elongation of at least 19%, are targeted.

Therefore, the purpose of the present invention is to provide such sheet and a method to produce it.

DETAILED DESCRIPTION

The present invention provides a method for producing a cold rolled steel sheet having a tensile strength TS of at least 1470 MPa and a total elongation TE of at least 19%, the method comprising the successive steps of:

-   -   annealing at an annealing temperature AT a cold rolled steel         sheet made of steel whose chemical composition contains in         weight %:     -   0.34%≤C≤0.40%     -   1.50%≤Mn≤2.30%     -   1.50≤Si≤2.40%     -   0%<Cr≤0.5%     -   0%<Mo≤0.3%     -   0.01%≤Al≤0.08%         the remainder being Fe and unavoidable impurities, the annealing         temperature AT being equal or higher than the Ac3 transformation         point of the steel, to obtain an annealed steel sheet,     -   quenching the annealed steel sheet by cooling it down to a         quenching temperature QT lower than the Ms transformation point         of the steel, typically between 150° C. and 250° C., to obtain a         quenched steel sheet, and,     -   making a partitioning treatment by reheating the quenched steel         sheet at a partitioning temperature PT between 350° C. and         450° C. and maintaining the steel sheet at this temperature         during a partitioning time Pt between 15 seconds and 150         seconds.

Preferably, the annealing temperature AT is between 870° C. and 930° C.

In two embodiments, after partitioning the steel sheet is cooled to the room temperature in order to obtain a non-coated steel sheet:

In the first embodiment, the composition of the steel is such that 0.36%≤C≤0.40%, Cr<0.05% and Mo<0.05%, the quenching temperature is between 190° C. and 210° C. and the partitioning time Pt is between 90 seconds and 110 seconds.

In the second embodiment, the composition of the steel is such that 0.34%≤C≤0.37%, 0.35%≤Cr≤0.45% and 0.07%≤Mo≤0.20%, the quenching temperature is between 200° C. and 230° C. and the partitioning time Pt is between 25 seconds and 120 seconds.

Preferably, the bare cold rolled steel is afterwards electro-galvanized.

In one embodiment, after partitioning the steel sheet is galvanized then cooled to the room temperature in order to obtain a coated steel sheet, the composition of the steel is such that 0.34%≤C≤0.37%, 0.35%≤Cr≤0.45% and 0.07%≤Mo≤0.20%, the quenching temperature is between 200° C. and 230° C. and the partitioning time Pt is between 25 seconds and 55 seconds.

The invention relates also to a coated or non-coated steel sheet made of steel whose chemical composition comprises in weight %:

-   -   0.34%≤C≤0.40%     -   1.50%≤Mn≤2.30%     -   1.50≤Si≤2.40%     -   0<Cr≤0.5%     -   0%<Mo≤0.3%     -   0.01%≤Al≤0.08%         the remainder being Fe and unavoidable impurities, the structure         comprising at least 60% of martensite and between 12% and 15% of         residual austenite, the tensile strength is at least 1470 MPa         and the total elongation being at least 19%.

In a particular embodiment, the steel sheet is non-coated, the composition of the steel is such that 0<Cr<0.05% and 0<Mo<0.05%, and the yield strength is higher than 1150 MPa.

In another embodiment, the steel sheet is non-coated, the composition of the steel is such that 0.35<Cr<0.45% and 0.07<Mo<0.20%, and the yield strength is higher than 880 MPa, the tensile strength is higher than 1520 MPa, and the total elongation is of at least 20%.

In another embodiment, the steel sheet is galvanized, the composition of the steel is such that 0.35%≤Cr≤0.45% and 0.07%≤Mo≤0.20%, the tensile strength is higher than 1510 MPa and the total elongation is at least 20%.

The invention will now be described in details but without introducing limitations.

According to the invention, the sheet is obtained by heat treating a hot or preferably a cold rolled non-treated steel sheet made of steel which chemical composition contains, in weight %:

-   -   0.34% to 0.40% of carbon to ensure a satisfactory strength and         improve the stability of the retained austenite. This is         necessary to obtain a sufficient elongation. If carbon content         is too high, the hot rolled sheet is too hard to cold roll and         the weldability is insufficient.     -   1.50% to 2.40% of silicon in order to stabilize the austenite,         to provide a solid solution strengthening and to delay the         formation of carbides during partitioning with appropriate         procedures to prevent the formation of silicon oxides at the         surface of the sheet which is detrimental to coatability.     -   1.50% to 2.30% of manganese to have a sufficient hardenability         in order to obtain a structure containing at least 60% of         martensite, a tensile strength of more than 1470 MPa and to         avoid having segregation issues which are detrimental for the         ductility.     -   0% to 0.3% of molybdenum and 0% to 0.5% of chromium to increase         the hardenability and to stabilize the retained austenite in         order to strongly reduce austenite decomposition during         partitioning. The absolute zero value is excluded due to         residual amounts. When the steel sheet is non-coated, the         molybdenum and the chromium can be eliminated and their contents         can remain less than 0.05% each. When the steel sheet is coated         by galvanizing, the molybdenum content is preferably from 0.07%         to 0.20% and the chromium content is preferably from 0.35% to         0.45%.     -   0.01% to 0.08% of aluminum which is usually added to liquid         steel for the purpose of deoxidation, preferably.

The remainder is iron and residual elements or unavoidable impurities resulting from the steelmaking. In this respect, Ni, Cu, V, Ti, B, S, P and N at least are considered as residual elements which are unavoidable impurities. Therefore, generally, their contents are less than 0.05% for Ni, 0.05 for Cu, 0.007% for V, 0.001% for B, 0.005% for S, 0.02% for P and 0.010% for N. Addition of microalloy elements such as Nb from 0 to 0.05% and/or Ti from 0 to 0.1% could be utilized to obtain the desired microstructure and an optimal combination of product properties

The non-treated steel sheet is a cold rolled sheet prepared according to the methods known by those who are skilled in the art.

After rolling the sheets are pickled or cleaned then heat treated and optionally hot dip coated.

The heat treatment which is made preferably on a continuous annealing when the sheet is not coated and on a hot dip coating line when the steel sheet is coated, comprises the following successive steps:

-   -   annealing the cold rolled sheet at an annealing temperature AT         equal or higher than the Ac3 transformation point of the steel,         and preferably higher than Ac3+15° C., in order to obtain an         annealed steel sheet having a structure completely austenitic,         but less than 1000° C. in order not to coarsen too much the         austenitic grains. Generally, a temperature higher than 870° C.         is enough for the steel according to the invention and this         temperature does not need to be higher to 930° C. Then the steel         sheet is maintained at this temperature i.e. maintained between         AT−5° C. and AT+10° C., for a time sufficient to homogenize the         temperature in the steel. Preferably, this time is of more than         30 seconds but does not need to be more than 300 seconds. To be         heated to the annealing temperature, the cold rolled steel sheet         is, for example, first heated to a temperature of about 600° C.         at a speed typically below 20° C./s then heated again to a         temperature of about 800° C. at a speed typically below 10° C./s         and eventually heated to the annealing temperature at a heating         speed below 5° C./s. In this case, the sheet is maintained at         the annealing temperature for a duration between 40 and 150         seconds.     -   quenching of the annealed sheet by cooling down to a quenching         temperature QT lower than the Ms transformation point between         150° C. and 250° C. at a cooling rate fast enough to avoid         ferrite formation upon cooling and preferably of more than 35°         C./second, in order to obtain a quenched sheet having a         structure consisting of martensite and austenite, then the final         structure contains at least 60% of martensite and between 12%         and 15% of austenite. If the steel contains less than 0.05% of         molybdenum and less than 0.05% of chromium, the quenching         temperature is preferably between 190° C. and 210° C. When the         steel sheet has to be galvanized and when the chemical         composition of the steel is such that 0.34%≤C≤0.37%,         0.35%≤Cr≤0.45% and 0.07%≤Mo≤0.20%, then the quenching         temperature is preferably between 200° C. and 230° C.     -   reheating the quenched sheet up to a partitioning temperature PT         between 350° C. and 450° C. The heating speed is preferably at         least 30° C./s.     -   maintaining the sheet at the partitioning temperature PT for a         partitioning time Pt between 15 sec and 150 sec. During the         partitioning step, the carbon is partitioned, i.e. diffuses from         the martensite into the austenite which is thus enriched.

Optionally, cooling the sheet down to the room temperature if no coating is desired or heating the sheet to a coating temperature, hot dip coating the sheet and cooling it down to the room temperature if a coating is desired. The hot dip coating is, for example, galvanizing, and the coating temperature is about 460° C. as it is known in the art.

The heating to the coating temperature is made preferably at a heating speed of at least 30°/s and the coating takes between 2 and 10 s.

Whether or not a coating is applied, the cooling speed to the room temperature is preferably between 3 and 20° C./s.

When the sheet is not coated and the steel contains preferably less than 0.05% of chromium and less than 0.05% of molybdenum, then the partitioning time is preferably between 90 sec and 110 sec. With such treatment it is possible to obtain sheets having a yield strength of more than 1150 MPa, a tensile strength of more than 1470 MPa and a total elongation of more than 19%.

When the sheet is not coated and the steel contains 0.35% and 0.45% of chromium and between 0.07% and 0.20% of molybdenum, then the partitioning time is preferably between 15 sec and 120 sec. With such treatment it is possible to obtain sheets having a yield strength of more than 880 MPa, a tensile strength of more than 1520 MPa and a total elongation of more than 20%.

When the sheet is coated the steel contains preferably between 0.35% and 0.45% of chromium and between 0.07% and 0.20% of molybdenum and the partitioning time Pt is preferably between 25 seconds and 55 seconds. In these conditions it is even possible to obtain coated steel sheet having a tensile strength higher than 1510 MPa and a total elongation of at least 20%.

As examples and comparison, it was manufactured sheets made of steels whose compositions in weight and characteristic temperatures such as Ac3 and Ms are reported in table I.

The sheets were cold rolled, annealed, quenched, partitioned and cooled to the room temperature or, galvanized after partitioning before being cooled to the room temperature.

The mechanical properties were measured in the transverse direction relative to the direction of rolling. As it is well known in the art, the ductility level is slightly better in the direction of rolling than in the transverse direction for such high strength steel. Measured properties are Hole expansion ratio HER measured according to the standard ISO 16630:2009, the yield strength YS, the tensile stress TS, the uniform elongation UE and the total elongation TE.

The conditions of treatment and the mechanical properties are reported in Table II for the non coated sheets and in Table III for the coated sheets.

In these tables, AT is the annealing temperature, QT the quenching temperature, PT the partitioning temperature. In Table II, GI is the temperature of galvanizing.

TABLE I Ref C Mn Si Cr Mo Al Ac3 Ms steel % % % % % % ° C. ° C. S180 0.29 2.02 2.44 0.004 Residual 0.059 920 290 (<0.003) S181 0.39 2.03 1.95 0.003 Residual 0.058 860 240 (<0.003) S80 0.36 1.99 1.95 0.41 0.088 0.045 850 250 S81 0.38 1.98 1.93 0.34 0.14 1.047 860 270

TABLE II AT QT PT Pt HE YS TS UE TE Example steel ° C. ° C. ° C. sec % MPa MPa % %  1 S180 920 240 400 10 — 982 1497 11.4 15.9  2 S180 920 240 400 100 17 1073 1354 13.9 19.9  3 S180 920 240 400 500 — 1082 1309 13.2 18.4  4 S181 900 200 400 10 — 1095 1583 12.5 13.8  5 S181 900 200 400 100 21 1238 1493 13.0 19.4  6 S181 900 200 400 500 — 1207 1417 13.1 17.7  7 S80 900 220 400 10 — 925 1658 9.4 9.4  8 S80 900 220 400 30 — 929 1603 15.1 20.5  9 S80 900 220 400 50 — 897 1554 16.1 21.1 10 S80 900 220 400 100 — 948 1542 18.1 21.4 11 S81 900 240 400 10 — 867 1623 8.1 9.3 12 S81 900 240 400 30 — 878 1584 11.4 11.8 13 S81 900 240 400 50 — 833 1520 10.8 12.2 14 S81 900 240 400 100 — 840 1495 15.9 17.3

TABLE III AT QT PT GI Pt HE YS TS UE TE example Steel ° C. ° C. ° C. ° C. sec % MPa MPa % % 15 S180 920 240 400 460 100 24 1127 1310 13.7 20.7 16 S181 900 200 400 460 10 — 933.4 1348 14.0 18.0 17 S181 900 200 400 460 30 — 1170 1425 13.8 20.1 18 S181 900 180 400 460 100 — 1353 1507 8.0 14.1 19 S181 900 200 400 460 100 19 1202 1399 13.0 20.2 20 S181 900 220 400 460 100 — 936 1280 14.3 18.0 21 S181 900 200 420 460 10 — 906 1346 11.2 10.6 22 S181 900 200 420 460 30 — 841 1298 14.7 19.3 23 S181 900 200 420 460 100 — 900 1322 14.5 19.1 24 S181 900 200 360 460 10 — 910 1357 14.5 19.0 25 S181 900 200 360 460 30 — 992 1356 14.0 18.9 26 S80 900 220 400 460 10 — 756 1576 10.5 11.1 27 S80 900 220 400 460 30 — 836 1543 18.3 20.3 28 S80 900 220 400 460 50 — 906 1534 18.6 21.6 29 S80 900 220 400 460 100 941 1394 8.1 8.58 30 S81 900 240 400 460 10 — 704 1518 6.6 6.8 31 S81 900 240 400 460 30 951 1438 8.9 8.9 32 S81 900 240 400 460 50 — 947 1462 13.5 18.5 33 S81 900 240 400 460 100 — 987 1447 15.7 19.6

The examples 1 to 14 show that it is only with the steel S181, which contains neither chromium nor molybdenum, and steel S80, which contains both chromium and molybdenum, that it is possible to reach the desired properties i.e. TS 1470 MPa and TE≥19%. In alloy S181, the desired properties are achieved for a quenching temperature QT of 200° C. and a partitioning time of 100 seconds. In this case, the yield strength is higher than 1150 MPa.

In alloy S80, which contains chromium and molybdenum, the desired properties are achieved for a quenching temperature QT of 220° C. and a partitioning time between 30 to 100 seconds (examples 7 to 10). In this case, the tensile strength is higher than 1520 MPa and the total elongation is more than 20%. Moreover, it is worth mentioning that all the examples containing Cr and Mo (7 to 14) have yield strengths significantly lower than the examples 1 to 6, concerning a steel without Cr and Mo.

The examples 15 to 33 show that only the examples corresponding to steels containing Cr and Mo are able to reach the desired properties when the sheets are galvanized (examples 27 and 28). For the steel S80, the quenching temperature has to be of 220° C. and a partitioning of 10 seconds is too short while a partitioning time of 100 seconds is too long. When the steel does not contain Cr and does not contain Mo, the tensile strength always remains lower than 1470 MPa. 

What is claimed is:
 1. A method for producing a non-coated steel sheet having a tensile strength TS of at least 1470 MPa and a total elongation TE of at least 19%, the method comprising the successive steps of: annealing at an annealing temperature AT a cold rolled steel sheet made of a steel having a chemical composition containing in weight %: 0.34%≤C≤0.40%, 1.50%≤Mn≤2.30%, 1.50≤Si≤2.40%, Cr<0.05%, Mo<0.05%, 0.01%≤Al≤0.08%, and a remainder, the remainder being Fe and unavoidable impurities, the annealing temperature AT being higher than an Ac3 transformation point of the steel, quenching the annealed steel sheet by cooling the annealed steel sheet down to a quenching temperature QT lower than an Ms transformation point of the steel and comprised between 190° C. and 210° C., and, making a partitioning treatment by reheating the quenched steel sheet at a partitioning temperature PT between 350° C. and 450° C. and maintaining the quenched steel sheet at the partitioning temperature PT during a partitioning time Pt comprised between 90 seconds and 110 seconds, after partitioning cooling the steel sheet to room temperature in order to obtain the non-coated steel sheet.
 2. The method according to claim 1 wherein the annealing temperature AT is from 870° C. to 930° C.
 3. The method according to claim 1, wherein the non-coated steel sheet is then electro-galvanized.
 4. The method according to claim 1, wherein the annealing temperature is greater than Ac3+15° C.
 5. The method according to claim 1, wherein the steel sheet is maintained at the annealing temperature for a holding time of greater than 30 seconds and less than 300 seconds.
 6. The method according to claim 5, wherein the holding time is from 40 to 150 seconds.
 7. The method according to claim 1, wherein the heating to the annealing temperature comprises first heating the cold rolled steel sheet to a temperature of 600° C. at a first heating speed below 20° C./s, then heating the cold rolled steel sheet to a temperature of 800° C. at a second heating speed below 10° C./s, then heating the cold-rolled steel sheet to the annealing temperature at a third heating speed below 5° C./s.
 8. The method according to claim 1, wherein the quenched steel sheet is reheated to the partitioning temperature PT at a heating rate of at least 30° C./s.
 9. The method according to claim 1, wherein the steel sheet is cooled to the room temperature at a cooling rate between 3 and 20° C./s.
 10. The method according to claim 1, wherein the non-coated steel sheet has a structure comprising, by volume, at least 60% of martensite and from 12% to 15% of residual austenite.
 11. The method according to claim 1, wherein the non-coated steel sheet has a yield strength greater than 1150 MPa.
 12. The method according to claim 1, wherein 0.36%≤C≤0.40%.
 13. A non-coated steel sheet made of steel having a chemical composition comprising in weight %: 0.34%≤C≤0.40%, 1.50%≤Mn≤2.30%, 1.50≤Si≤2.40%, Cr<0.05%, Mo<0.05%, 0.01%≤Al≤0.08%, and a remainder, the remainder being Fe and unavoidable impurities, the non-coated steel sheet having a structure comprising, by volume, at least 60% of martensite and between 12% and 15% of residual austenite, and the non-coated steel sheet having a tensile strength of at least of 1470 MPa and a total elongation of at least 19%.
 14. The non-coated steel sheet according to claim 13, wherein the non-coated steel sheet has a yield strength greater than 1150 MPa.
 15. The non-coated steel sheet according to claim 13, wherein 0.36%≤C≤0.40%. 